This invention relates to low-density polyurethane foams and methods of manufacture thereof, as well as articles formed using low-density polyurethane foams. The foams are of particular utility in the manufacture of a sealing member, especially a sealing member suitable for being disposed between the margin of a liquid crystal display screen in the display of portable, telecommunication equipment such as a portable telephone or the margin of a microphone or the like and the internal surface of a case thereof.
Within electrical machinery and apparatus such as a television set, a computer, a portable telephone, or a display of a personal digital assistant, an internal apparatus such as a liquid crystal display, a microphone or a speaker is generally arranged such that the internal apparatus faces the outside of its casing. Use of a sealing member between the edge of the internal apparatus and the casing improves dust control, prevents leaking of light from a backlight and the like, and the prevention impact damage.
For many applications, including small devices exemplified by recent portable telephones, an increase and sophistication in function and a decrease in weight have been desired. Based on this, a performance requirement for the sealing member above has become more sophisticated. For example, the above increase and sophistication in function for recent portable telephones have been carried out by attaining an increase in the integration of parts via unification of the IC size to be used, the saving of space by introducing a very large-scale integration (VLSI), and the multi-layering of a substrate. When many parts are integrated within the casing of a portable telephone as described above, the internal structure thereof, for example, the interlocking structure of the casing becomes more complicated. Gaps between the internal parts and the casing can form, requiring a sealing member arranged between the internal parts and the casing. Sealing members must, however, be able to provide an adequate seal at a wide range of compression ratios (from low to high), corresponding to variations in the gap size to be filled.
Generally the sealing properties of a sealing member are evaluated as to whether the sealing member is easily deformed by a load or not. Therefore, in order to have adequate sealing properties when the sealing member is highly compressed, a low-density soft polyurethane foam, an olefin foam such as polyethylene foam or polypropylene foam, or a rubber foam, or the like is used. However, soft, thin foams are often obtained by slicing sheets of the required thinness after the foam has been obtained in the form of a slab. Such foams thus have no skin layer and poor adhesive properties. Soft foams therefore are not as dust-proof or as light-blocking as might be desired. Foams having a skin layer can be also produced from the above raw materials, but the compressive residual strain of such sealing members is large, and the foams do not have stable sealing properties over time. Furthermore, since such foams have a large cellular diameter, it causes problems in dust-proof performance and light-blocking performance.
In addition, the casings of various electrical machinery and apparatuses use lightweight and high-strength material of generally high conductivity, for example, magnesium alloys or the like. In order to effectively avoid disadvantages such as the generation of electromagnetic waves based on this, sealing materials having a lower dielectric constant, i.e., materials having high insulating properties are desired. However, since the dielectric constant is essentially a material-intrinsic value, it is difficult to select a material having both sealing properties and a low relative dielectric constant.
Japanese Patent Laid-Open Publication No. 2001-100216 specifically discloses a polyurethane foam as a sealing member. This polyurethane foam is formed according to a mechanical froth method, and easily deformed with a low load at a 25% compression. A plastic film as a back-up material is integrally formed on a single side of the foam. In the mechanical frothing method, polyurethane foams are formed by mixing an organic isocyanate component, an active hydrogen-containing component(s), a surfactant, and a catalyst, then dispersing an inert gas throughout the mixture by mechanical beating of the mixture to form a heat curable froth. The froth is then cured to form the foam.
Polyurethane foams can also be formed by physical or chemical blowing of the isocyanate component, active hydrogen-containing component mixture, and other additives, followed by subsequent curing. Each of these methods of foam production has a number of drawbacks, especially when thin polyurethane foam sections having a low and uniform density across the entire cross-section are required. For example, foams made by mechanically frothing a polyurethane liquid phase using air or inert gas often have a uniform cell structure with good physical properties. However, such foams also have an undesirably high density, on the order of greater than 15 pounds per cubic foot (pcf) (240 kilogram per cubic meter (kg/m3)). The foams are also very difficult to manufacture in thicknesses of example less than 0.5 inches (13 millimeters (mm)), preferably less than 0.2 inches (5 mm), and particularly less than 0.12 inches (3 mm). On the other hand, foams made by physical or chemical blowing can have very low cell densities, on the order of 2 pcf (32 kg/m3), but also have irregular cell structure. Thin sections made in this manner often have poor physical properties.
In view of these drawbacks, there exists a need in the art for soft foams having a thickness less than 0.5 inches (13 mm), preferably less than 0.12 inches (3 mm) and low densities, on the order of up to 25 pcf (up to 400 kg/m3), preferably 3 to 15.6 pcf (50 to 250 kg/m3), together with uniform cell structure.